Add missing info in readme

[apirrone]

• [x] Need git-lfs
• [x] Recommandation: work in virtual environment
• [x] Define what is a zero shot model
• [x] Fix invisible text in logo when github not in dark mode
• [x] Before "get started in few lines of code", indicate that the user can try the models in colab or on our HF space
• [x] Show how to get webcam stream using cv2 in the example (so that it is just a copy and paste to try)
• [x] Remove pip install commands from Pypi